The minimum size of your own background image should be 1080 x 380 pixels, the minimum size of your profile image should be 800 x 800 pixels.
Images with a lower pixel rate are not upscaled. Only images with a higher pixel rate are proportionally downscaled, until they eventually reach the height and the width of the frame. Which means that an image with lower resolution pixilates, appears with a grey frame or can have the effect of being cut off.
Same thing applies to the profile image, moreover, it is cut out in the middle.
The maximum size of an image should be 5 MB, to be uploaded properly.